What does 'date modified mean on an adobe acrobat document?
In most cases, "date modified" refers to the date a file's
contents (the information in the file) were changed. For example,
let's say you create a document on July 5, 2013, but later, you
come back to it and revise it-- make changes to it in some way. The
document will show that it was modified (changed) on that date.
Also, sometimes, Adobe will provide an updated version of a
document, and the "date modified" refers to when they made the
changes in the update.